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Puzzle over declining male births

Where are the boys going? ... scientists are at a loss to explain
the changing sex ratio of newborns.Photo: Rob Homer

June 3, 2008 - 2:38PM

A puzzling phenomenon is playing out across the world, in
countries as diverse as the United States, Sweden and Japan.

Though more boys are being born than girls in most places, their
numbers are falling. And no one is sure why.

The change is small, but real. In the US, the number of baby
boys versus girls has been declining since 1970, translating into
17 fewer males for every 10,000 births or an estimated 135,000
fewer boys born between 1970 and 2002, according to a study last
year in Environmental Health Perspectives.

Some experts suggest the shift is part of a naturally occurring,
cyclical pattern in population dynamics. But others think a notable
change is under way, driven by factors such as environmental
contaminants and various types of stress, such as economic
hardship.

These issues could affect boys more because they're actually the
weaker sex - more vulnerable than girls to illness and death from
conception to grave.

Nature's way of compensating is to produce more males than
females, increasing the likelihood that the sexes will survive to
reproductive age in equal numbers. But recent decades have eroded
the gap between the sexes.

The difference may seem tiny, but "it's important to look at the
really big picture here, which is that there are global indications
that something unusual is going on," said Devra Davis, director of
the Centre for Environmental Oncology at the University of
Pittsburgh and lead author of last year's report.

The sex ratio is an indicator of population health, and
unexpected changes could be an important signal that people are at
risk biologically, she said.

Several Latin American nations have reported a similar shift in
the sex ratio at birth, as have Finland, Norway, Wales and the
Netherlands. Late last year, several Arctic communities documented
a startling decline in the number of boys being born. Studies have
shown changing sex ratios in Italian cities and among fish-eating
women in the Great Lakes region.

None of these countries or areas has a tradition of sex
selection, which in any case usually favours boys.

The puzzling phenomenon has inspired a flurry of research on
what could be causing the population shifts. Davis' hypothesis is
that "there is something happening after conception that is making
it harder for boys to exist in the maternal foetal
environment."

A growing body of research indicates that could include exposure
to pollutants such as pesticides, mercury, lead and dioxin. More
controversial is the idea that synthetic chemicals known as
endocrine-disrupters may be damaging male foetuses during critical
periods of development or affecting men's sperm counts and
testosterone levels.

That thesis is "very interesting and provocative" but hasn't
been proved, said Dr Rebecca Sokol, past president of the Society
of Male Reproduction and Urology.

The steepest sex ratio declines observed in the world have
occurred on the 1,214 hectare Aamjiwnaang (AH-jih-nahng) First
Nation reservation in Canada.

The number of boys versus girls there began dropping in the
early 1990s, according to data published in 2005 in
Environmental Health Perspectives. Between 1999 and 2003,
researchers found, only 46 boys were born out of 132 recorded
births.

"You get angry and you get worried, thinking what could be
causing this," said Ada Lockridge, a member of the tribe who
compiled the data and has since become an activist. "And then you
want to learn more."

Dozens of petrochemical, polymer and chemical plants surround
the reservation on three sides. Mercury and PCBs contaminate the
creek that runs through the land, and air-quality studies show the
highest toxic releases in all of Canada, said Jim Brophy, executive
director of Occupational Health Clinics for Ontario Workers, based
in Sarnia, the nearest city.

Still, there is no proof that pollution is responsible, and data
from surrounding Lambton County don't show a similar impact. The
findings represent a "short period of time and a small population"
and require further study, said Dean Edwardson, general manager of
the Sarnia-Lambton Environmental Association, which represents area
industry.

Indeed, there's strong evidence from other areas that men
exposed in the workplace to pesticides, lead and solvents and in
industrial accidents to toxic substances such as dioxin end up
fathering fewer boys.

When a July 1976 chemical plant explosion in Seveso, Italy sent
a cloud of dioxin over the area, researchers discovered that no
boys were born for seven years to parents who had the highest
levels of the toxin in their blood.

In another study, men exposed to the pesticide
dibromochloropropane fathered three times as many daughters as
expected.

Some evidence also suggests stress can reduce the motility or
viability of Y-bearing sperm, reducing the likelihood that boys
will be conceived. This may help explain why fewer boys are born
after natural disasters such as earthquakes and floods - a finding
well documented in the scientific literature.

Mothers are thought to have a different set of responses to
stress, which also could favour girls over boys.

When pregnant women struggle with adverse circumstances -
economic hardship, poor food supply - a biological mechanism that
"culls" weak male foetuses may be inadvertently deployed, said
Ralph Catalano, a professor of public health at the University of
California, Berkeley.

From an evolutionary standpoint, this would make sense, since
boys require more parental effort to raise while also dying at a
higher rate, Catalano explained. When times are tough, it's more
advantageous to give birth to a girl, he said.

Among Catalano's thought-provoking findings: The number of boys
born in New York City relative to girls fell significantly after
the September 11 terrorist attacks.

The world's leading expert on the science of sex ratios, William
H James, who spent his career at University College in London,
offers another possible explanation: Hormones in both parents at
the time of conception affect the sex of offspring.

Higher levels of testosterone and oestrogen are associated with
the birth of sons, James says, while elevated levels of
gonadotropins and progesterone are associated with daughters. These
hormones are internally regulated but also are subject to external
influences such as alcohol, cigarette smoke, radiation, chemicals,
and illnesses in parents.

A spinoff of the theory is the notion that the timing of
conception can help determine the sex of offspring because hormone
levels fluctuate during a woman's fertile period.

James observed that women who conceive early or late in their
fertile periods are more likely to have boys. Couples who have lots
of sex have a higher probability of bearing sons, he concluded,
because they're more likely to conceive early on, he concluded.